I only just did this earlier, went onto the gapps section on the cyanogen wiki (can't link, new user) on the browser on my moto droid 4 and downloaded the appropriate gapps zip via vends.
Assuming you used Safestrap or something along the lines of that to flash cyanogen you just go back into the safestrap boot menu button screen thing and using the install function, install the gapps zip that should be in the downloads folder of your sdcard and voila!
